1. Field of the Invention
The present invention relates to data processing electrical circuits, and more particularly to a high-speed bipolar logarithmic analog-to-digital (A/D) converter to provide a signed binary number output proportional to the logarithm of a bipolar analog input signal.
2. Description of the Prior Art
Prior art circuits are illustrated in FIGS. 1(a) and 1(b) which illustrate one method for providing a true logarithmic output suitable for arithmetic use. Generally, a current-to-voltage transducer with a mathematically true logarithmic response is used, usually the base-emitter junction of a transistor, within the limited range of its "ideal" behavior. In FIG. 1(a) a direct current offset is used to accomodate a bipolar input with a single logging transistor converter. The frequency response of this circuit is unsymmetrical since the amplifier responds more slowly at the lower circuit (positive) input. The circuit also places high requirements on the components for a given accuracy level because of the wide response required of the logging transistor and the use of multiple reference currents and voltages. It also is difficult to accurately compensate for temperature variations in the circuit gain. Alternatively, the more complex circuit of FIG. 1(b) inserted at Z in FIG. 1(a) overcomes some of these limitations, but the multiple operational amplifiers make it slow to respond. It also is difficult to compensate for gain variations with temperature.
Another method involves piecewise linear approximation of a hyperbolic sine function by a large resistor network. This method requires large scale integrated circuit technology for implementation. It is suitable for digitally encoding and decoding low speed communications signals, but it is not suitable for arithmetic operations.